The present invention relates to rigid polyurethane foams and to surfactants used in such foams. The term polyurethane as used herein is intended to include so-called isocyanurate foams, which include a greater proportion of isocyanate than normal polyurethanes, allowing trimer formation. Both polyester and polyether urethanes are also encompassed by the term.
Surfactants are used in rigid polyurethane foam formulations to facilitate foam formation. Silicone surfactants, siloxane-polyoxyalkylene block copolymers, are almost exclusively used as surfactants for rigid polyurethane foam systems. The general formula for such copolymers is as follows: ##STR1## where ME is CH.sub.3 and
R is a terminal group compatible with surfactant activity. Example R groups include acetoxy group (OAc), a hydroxyl group (OH), a methoxy group (OME), dodecyl succinic anhydride (DDSA), carboxy functional groups and amino functional groups. PA1 x=7 to 10 PA1 y=2 to 5 PA1 m=5 to 15 PA1 n=0 to 5
Not all siloxane-polyoxyalkylene copolymers work satisfactorily as surfactants for rigid polyurethane foams. For rigid polyurethane foams it is generally preferred that siloxane-polyoxyalkylene copolymers have the following parameters:
For rigid polyurethanes, the silicone backbone must usually have a molecular weight of from about 1000 to 7000. For flexible urethanes, the silicone backbone molecular weight runs from about 7000 to 12000. For the high resiliency urethanes, the lower silicone backbone molecular weights are preferred, i.e., 220 to 1000.
Many organic surfactants are substantially less expensive than silicone surfactants. Recently, some specific types of organic surfactants have been used as the exclusive surfactant in rigid polyurethane formulations. However, most common organic surfactants are rarely used in rigid polyurethane foams. Among various problems encountered, organic surfactants generally substantially degrade the insulating capability of the rigid polyurethane foam. Typical K factors for rigid polyurethanes incorporating organic surfactants exceed 0.125, as exemplified in the following examples:
______________________________________ Organic Surfactant Incorporated Into A Rigid Polyurethane Foam System K Factor ______________________________________ Triethanolamine salt of a linear alkyl .149 benzene sulfonic acid Alkyl (C.sub.12 -C.sub.14 -C.sub.16) dimethyl amine oxide .144 Cocoamidopropyl betaine .143 Sodium octyl sulfate .144 Triethanolamine N--lauryl sarcosinate .147 Tridecyl alcohol poly ethoxy sulfate .142 Sodium dioctyl sulfosuccinate .132 Linear primary alcohol ethoxylate .137 Sodium C.sub.14 -C.sub.16 olefin sulfonate .147 Sodium dodecyl benzene sulfonate .168 ______________________________________
These compare with typical K factors of 0.115 for rigid polyurethanes incorporating silicone surfactants.
Both organic surfactants and silicone surfactants are used in formulating flexible polyurethane foams. Union Carbide U.S. Pat. No. 3,594,334 to Marlin discloses incorporating combinations of anionic organic surfactants and siloxane-polyoxyalkylene surfactants (of 600 to 17,000 molecular weight containing 14 to 40% siloxane and containing 75% oxyethylene in the oxyalkylene component) as surfactants in flexible polyurethane formulations. Such surfactant combinations would not be expected to operate satisfactorily in typical rigid urethane formulations because of the adverse impact of the organic surfactant on the K factor, as well as other factors. My experimental work supports this expectation.